Chemistry Reference
In-Depth Information
bubbler. Inert gas is then passed through the vessel and out the bubbler at a con-
venient flow rate. Again, ethylene will be liberated and the viscosity of the reac-
tion mixture will increase as a sign that ADMET is proceeding. Additional catalyst
may be added as desired, and samples for GPC or other analyses may be conveni-
ently taken if necessary. It may be advantageous to finish the reaction with vacu-
um to promote higher conversion. The reaction is worked up as above.
6.7.3
ADMET with Volatile Monomers
If a volatile monomer is used in an ADMET polymerization, a condenser should
be used to return monomer vapor to the reaction mixture. The kinetics of step-
growth polymerization dictate that the concentration of monomer falls very
quickly to produce dimer, trimer, and so forth. Nevertheless, monomer will be
present for some time after the start of the polymerization. If the monomer is
particularly volatile, a dry ice-isopropanol condenser is useful. This can be con-
structed in any glass shop by attaching a cup-shaped cooling reservoir to a vacu-
um valve or other cylindrical glass tube with the required joints and valve. If the
monomer is only slightly volatile, or the carrier gas method is used, a water-
cooled condenser is sufficient to retain monomer in the flask, while allowing eth-
ylene to escape.
6.7.4
Viscous or Solid Monomers or Polymers
Liquid ADMET monomers often become so viscous that magnetic stirring is im-
possible. There are several solutions to this. One is to attach a strong horseshoe
or other magnet to a mechanical stirrer and rotate it very slowly under the vessel
and temperature bath to slowly agitate the polymer melt. Another solution is to
use a high viscosity mechanical stirrer if the polymer is a viscous melt. Vacuum
adapters for the stirring shaft are available, so that the vacuum method may be
used with mechanical stirrers.
If the polymer solidifies upon polymerization, solvent may be added to disperse
the polymer. If the monomer is a solid, solvent may be used from the beginning
of the reaction. Solvent may be added through a Teflon valve or into an extra neck
containing a septum via syringe or cannula. Schlenk technique should be used
throughout. A vacuum may then be periodically applied. Ethylene and solvent will
both distill out upon application of high vacuum. A moderate vacuum (30 torr, for
example) may also be used with a high boiling solvent and water-cooled condens-
er to remove ethylene and retain most of the solvent in the vessel. The carrier gas
method, with no vacuum, also works in concentrated solution.
It is recommended that the least amount of solvent required to disperse the sol-
vent be used to avoid formation of cyclic oligomers. Also, if applying vacuum to a
solution ADMET polymerization, the vacuum should be applied very carefully to
prevent bumping.
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